The present invention is directed to new devices and methods for actuating a sucker rod pump. Many methods of actuating sucker rods have been proposed. While their use is not limited to only oil wells, sucker rods are particularly well adapted for use therein. It follows that many methods of actuating sucker rods for oil wells have also been proposed.
Of all the methods of actuating sucker rods for pumping oil previously proposed, the walking-beam surface unit is the most utilized device for actuating sucker rods because of its superior efficiency. This device, a representation of which is set forth in FIG. 1, typically utilizes an electric rotary motor 24 in conjunction with a speed-reduction gear box 26 to oscillate a walking beam 34 by means of a crank 27 and pitman 32. This construction converts the rapid rotational motion of the motor 24 into the relatively slow, reciprocating or oscillating motion of the walking-beam 34. The oscillating motion of the walking-beam 34 is transferred to the polished rod 10 and so to the subsurface pump 16, by means of a hanger cable 36 suspended from the end of the walking beam 34 opposite the crank 27 and pitman 32. The result is oil pumped at relatively low operating and capital costs.
This technology has been ubiquitous in oil fields for decades. See Lester Charles Uren, Petroleum Production Engineering, McGraw-Hill Book Company, Inc. New York (1939). Yet, despite the walking-beam surface unit being one of the most efficient devices heretofore available for actuating a sucker rod pump, the design is nevertheless inherently inefficient and inflexible. The present invention is directed to actuating a sucker rod pump with much greater efficiency than this decades-old technology.
With respect to the walking unit's inefficiencies, the electrical rotary motor 24, used as a prime mover, operate efficiently: at about 85% efficiency. However, the mechanical conversion of its high-speed, rotary motion by the speed-reduction gear box 26 to the slow, reciprocating motion needed to actuate the rods results in significant energy losses. The energy loses include friction losses in the gear box. Indeed, the gear box is only about 50% efficient. Other energy loses include friction losses in the bearings associated with the crank, pitman, and walking-beam.
In addition to the mechanical inefficiencies in the walking-beam surface unit, it has substantial design limitations which translate into high operating costs, operating costs which are substantially reduced, if not eliminated, by the present invention. For example, as FIG. 1 demonstrates, the walking-beam surface unit has a significant amount of articulation. As with most highly articulated mechanical devices, this conventional walking-beam surface unit tends to fail on a frequent basis. Moreover, this tendency of highly articulated mechanical systems, such as the walking-beam surface unit, to break down is exacerbated by severe environments. As significant amount of oil production occurs in the desert (Middle East), in the tundra (Alaska, Siberia) and on the open ocean (North Sea), it follows that the maintenance difficulties, and associated costs, are legion.
In addition to the likelihood of the walking-beam surface unit failing, the remoteness of such units adds an additional dimension to the cost of maintaining them. The units must be constantly tended to by manned crews to prevent lost production from unit failure. The prior art system has no capability to provide an operator with a remote diagnosis, adjustment or maintenance. Routine, manned maintenance is the only manner to insure that the pumps are working and working to their maximum potential.
Another design limitation of the walking-beam surface unit which adds to its cost of operation is the inflexible nature of its operation. Adjustments of the stroke length of the sucker rod are required when a well is set up and periodically thereafter. Unfortunately, such adjustments are difficult to make on the conventional walking-beam surface unit. Stroke-lengths can be varied through a typical range of only six fixed increments. Any time the stroke length is changed, the rig must be dismantled. Dismantling the heavy iron rig to vary the position of the pitman connection in the crank arm is tedious, labor-intensive work.
In addition to the difficulty in adjusting the stroke length, there is no easily operable facility on the conventional walking-beam surface unit for adjusting the speed, measured in strokes per minute, of the sucker rod pump. The ability to easily adjust the speed of the sucker rod pump is a desirable feature as it would facilitate operating the sucker rod pump in a variety of conditions. Also, it is desirable to pump some wells intermittently, yet the design of the conventional walking-beam surface unit does not facilitate intermittent operation, since the mechanical system tends to "freeze" once stopped and it is difficult and time-consuming to restart it.
In addition to the difficulty in making adjustments in the walking-beam surface unit, the fixed and non-adjustable, adjustable, sinusoidal deceleration/acceleration profile of the conventional walking-beam surface unit is not optimum for the operation of a sucker rod pump. The conventional profile imposes shock loading on the system, stressing the gear box and associated power train and results in excessive sucker rod vibration and consequently in accelerated rates of surface unit and sucker rod wear and failure.
Another significant disadvantage of the conventional walking-beam surface unit design is its use of a flexible hanger cable 36 to attach the polished rod 10 to the walking beam 34 (See FIG. 1). Lack of a rigid connection between the surface unit of an oil pumping rig and the sucker rod string invites excessive vibration and rapid stress variation in the sucker rod string.
Various modifications have been proposed to eliminate some of the difficulties discussed above with the conventional walking-beam surface unit. However, they all have resulted in undesirable compromise: they have addressed some of the problems but only at the expense of overall system efficiency.
Recently, a linear motor has been proposed as a replacement for the rotary motor, gearbox, and crank of conventional walking-beam surface units. As disclosed in U.S. Pat. No. 5,409,356 issued to Massie, it has been suggested to use a linear motor to oscillate the walking-beam of a conventional type surface unit, thus eliminating the rotary motor, gearbox, crank, and pitman. This modification of a conventional walking-beam type surface unit is aimed primarily at removing the inefficiency of the gearbox and giving the possibility of easy stroke length and acceleration/deceleration profile adjustment.
A main feature of the well pumping system disclosed in Massie is its improved flexibility due to the use of a linear motor to oscillate the beam. For example, stroke-length adjustments required in setup and ongoing operation of oil pumping units are easily afforded with a linear motor. Necessary adjustments in pumping speed (strokes per minute) are also easily accomplished in the Massie device. Finally, the linear motor-driven beam pumping system is relatively well-suited to the need for intermittent operation, since it lacks a gearbox, crank, and pitman and their attendant restarting difficulties.
However, the adaptation of a linear motor to a conventional walking-beam design as proposed by Massie results in a very inefficient system in terms of power consumption. The Massie device employs an electro-regenerative system as the primary means of static load counterbalancing. In this system, the linear motor would be operated as a generator on the down stroke of the sucker rods, the energy of the falling rod string thus producing electric power. This electrical energy is then stored in a battery or similar device. On the upstroke, when the machine is lifting the dead weight of the rod string and oil column, the linear motor draws upon the energy stored in the battery.
This type of counterbalancing system is very inefficient due to the accumulated energy losses incurred in generation, switching, storage, retrieval and finally, motor losses. Since the static load in a sucker rod system is usually great, efficient counterbalancing is critical. Energy efficiency of the Massie device would therefore appear to be seriously compromised.
Whereas Massie proposes a modification of a conventional walking-beam surface unit to include a linear motor, the present invention comprises a completely new surface unit design and concept which solves the problems of the prior art.